62 Allowance for Contraction.
Contraction of Castings.—This is a subject involving
both thought and practice, and although a few general rules can
be given, success depends on very many points. It has been
previously mentioned that the moulder raps the rod that draws
the pattern from the sand. This rapping taking place in a
horizontal direction, it is evident that the sides of the mould
only are affected by it.
The pattern maker must not only take account of this, but
also of the particular moulder he has to deal with, for some
moulders lift a pattern with less rapping than others. In small
castings, up to about six inches across, the enlargement of the
mould by rapping will be about compensated by the shrinkage of
the casting; but in large moulds, the amount of .shrinkage will be
so much greater than the effect of rapping, that the latter may
be entirely overlooked, account being taken only of the in-